The present invention relates to a rubber composition which is cured by heating into a rubbery elastomer having excellent mold-releasability, thermal stability, anti-freeze resistance and anti-weathering resistance along with excellent mechanical strengths and electric properties not only at ordinary temperatures, but also at elevated temperatures as well. In particular, the invention relates to a rubber composition comprising a blend of natural rubber or an organic synthetic rubber and a silicone rubber capable of forming a cured rubbery elastomer.
Several types of commercially available silicone rubbers curable by heating into rubbery elastomers are hitherto known. The most conventional types are silicone rubbers curable by crosslinking with an organic peroxide and those curable by addition reaction to crosslink formation between aliphatically unsaturated organic groups and silicon-bonded hydrogen atoms in the presence of platinum or a platinum compound as a catalyst to accelerate the addition reaction.
It is well known that silicone rubbers are generally much better than other organic synthetic rubbers in their heat resistance and low-temperature behaviors. The silicone rubbers, however, are disadvantaged by their relatively poor mechanical properties and expensiveness, compared to the other organic synthetic rubbers. Therefore, it has been one of the largest requirements in rubber technology to obtain a rubber composition capable of giving elastomers which have the same excellent heat resistance and low-temperature behaviors as the silicone rubbers, and also the same high mechanical properties as ordinary organic rubbers. Such requirement naturally led to an attempt to blend the silicone rubber with the organic rubber.
It is an established knowledge in the art, however, that co-vulcanization of a blend of a silicone rubber of the above-mentioned types, with natural rubber or an organic synthetic rubber, is very difficult and can hardly give a rubbery elastomer with superior properties. For example, an ethylene-propylene copolymeric rubber was blended with an alkenyl-containing organopolysiloxane and subjected to co-vulcanization with an organic peroxide as the vulcanizing agent. However, the thus-obtained elastomers had rather low tensile strength, say, around 30 to 45 kg/cm.sup.2, far lower than the values possessed by organic synthetic rubbers (see U.S. Pat. No. 3,227,777). The inventors of the present invention had previously proposed a rubber blend composed of an organic rubber and a mercapto group-containing organopolysiloxane in a copending U.S. patent application Ser. No. 877,822, though their blend had insufficient improvements in the mechanical properties and thermal stability.
A blend of natural rubber or an organic synthetic rubber and a silicone rubber of the peroxide-cure type can be co-vulcanized, but the resultant products will come to possess remarkably lowered thermal stability and physical properties, e.g. mechanical strengths, so that they are hardly suitable for practical use.
In a blend of natural rubber or an organic synthetic rubber with a silicone rubber curable by the mechanism of the addition reaction, on the other hand, the catalytic activity of the platinum catalyst is liable to be lost by poisoning with trace amounts of various compounds formulated in the above-mentioned organic rubber, such as sulfur compounds, amine compounds and phosphorus compounds, as well as compounds of metals, e.g. tin, zinc, bismuth, cobalt and the like, resulting in a decreased practical value of the rubber blend compositions.